In engines of the above type the difference AP in the gas pressure between the means supplying the engine with air at pressure P and the engine combustion gas exhaust system is relatively low and in practice is imposed by the specifications of the supercharged air supply means.
Scavenging can take place only during a limited part of each cycle, another and large part of the cycle being devoted to compression and expansion of the gases renewed in the chamber.
As a result the geometry and the operation of the inlet and exhaust valves play a decisive role in the efficiency, the power and the speed of the engine.
Increasing the size of the valves rapidly runs up against a geometrical limit imposed by the dimensions of the cylinder head while increasing the valve lift, that is to say the distance the valve moves away from its seat, and the lift speed, which are determined by the profile of the cams opening and closing the valves, is rapidly limited by mechanical constraints imposed by the permissible contact pressure between the nose of the cam and the components that it actuates.
This limits performance, i.e. the permeability of the cylinder head, the efficiency of use of the air passing through the cylinder head, i.e. the ratio between the mass of air enclosed in the working chamber at the end of scavenging to the mass of air passing through the cylinder head, and the scavenging efficiency, i.e. the ratio between the mass of air and the total mass of gas enclosed in said chamber at the end of scavenging.
Patent application EP-A-0 673 470 (or U.S. Pat. No. 5,555,859 or WO 95/08052) has made it possible to improve significantly on the above limitations by providing a single inlet valve and a single exhaust valve,
This arrangement optimises and controls scavenging and doubles the actual lift of the exhaust valve because the inlet and exhaust valves lift in opposite directions.
If means are provided to cause the inlet air passing through the inlet valve to rotate, axi-symmetrical centrifugal layering can be achieved and the fuel can be injected into a hot central area that is relatively impoverished in oxygen, to obtain the advantages described in the above patent and in patent application FR-A-2 690 951.